Magnetic shape memory
Encyclopedia
Magnetic shape-memory alloys (MSMAs), or ferromagnetic shape-memory alloys (FSMAs), are ferromagnetic materials which exhibit large strains under the influence of an applied magnetic field
due to martensitic phase transformation
. Magnetic shape-memory alloys, with near-stoichiometric Ni2MnGa being the most studied example, differ from other magnetostrictive materials, such as Terfenol-D and Galfenol, as they produce much larger strains by twinning, sometimes as large as 9%, under relatively low bias magnetic fields. The mechanism is based on the magnetic anisotropy of the material.
MSMAs produce a similar phase transformation between martensite 1 and martensite 2 (the two variants), as other shape memory alloys (SMAs) which change phase between austenite and martensite with the application of thermal energy. Few models have been developed which describe the constitutive response of MSMAs. Typically, thermodynamic modeling is used to describe the materials behavior.
When finding strain for MSMA the total strain equals the sum of the parts:
εtotal = εelastic + εreorientation
where εreorientation is defined as
εreorientation = εr,max*ξ.
ξ and εr,max are defined as variant two volume fraction and maximum reorientation strain, respectively.
ξ may be found analytically from a driving force function found from gibbs free energy using the relations of a polynomial or trigonometric hardening function. Variant 2 volume fraction (the variant which expands the specimen when exposed to magnetic energy) is a function of the magnitude of bias magnetic field, applied stress, heat, magnetic anisotropy energy, and other material properties such as magnetization saturation.
Due to the nature of MSMA, a shift in the direction of magnetization is produced when applying a stress to a fully strained element exposed to a bias field. The magnitude of this shift is dependent on the strength of the applied field and material properties. Using faradays law of induction, it is evident that MSMAs may be used for energy harvesting using a pickup coil, or inductor.
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
due to martensitic phase transformation
Diffusionless transformations
A diffusionless transformation is a phase change that occurs without the long-range diffusion of atoms but rather by some form of cooperative, homogeneous movement of many atoms that results in a change in crystal structure. These movements are small, usually less than the interatomic distances,...
. Magnetic shape-memory alloys, with near-stoichiometric Ni2MnGa being the most studied example, differ from other magnetostrictive materials, such as Terfenol-D and Galfenol, as they produce much larger strains by twinning, sometimes as large as 9%, under relatively low bias magnetic fields. The mechanism is based on the magnetic anisotropy of the material.
MSMAs produce a similar phase transformation between martensite 1 and martensite 2 (the two variants), as other shape memory alloys (SMAs) which change phase between austenite and martensite with the application of thermal energy. Few models have been developed which describe the constitutive response of MSMAs. Typically, thermodynamic modeling is used to describe the materials behavior.
When finding strain for MSMA the total strain equals the sum of the parts:
εtotal = εelastic + εreorientation
where εreorientation is defined as
εreorientation = εr,max*ξ.
ξ and εr,max are defined as variant two volume fraction and maximum reorientation strain, respectively.
ξ may be found analytically from a driving force function found from gibbs free energy using the relations of a polynomial or trigonometric hardening function. Variant 2 volume fraction (the variant which expands the specimen when exposed to magnetic energy) is a function of the magnitude of bias magnetic field, applied stress, heat, magnetic anisotropy energy, and other material properties such as magnetization saturation.
Due to the nature of MSMA, a shift in the direction of magnetization is produced when applying a stress to a fully strained element exposed to a bias field. The magnitude of this shift is dependent on the strength of the applied field and material properties. Using faradays law of induction, it is evident that MSMAs may be used for energy harvesting using a pickup coil, or inductor.